Thin film transistors (TFTs) have low power consumption, small sizes, and mature fabrication technology, and are widely used in display devices. A TFT having a great channel length often provides a higher turn-on current, and the TFT often has better performance. The channel length of a TFT refers to the length of the channel between the source electrode and the drain electrode.
For a display device with high resolution, TFTs with small sizes are desired. Accordingly, requirements on the fabrication, the electrical properties, and the reliability of the TFT array substrate are more demanding. Particularly, in organic light-emitting diode (OLED) display technology, a driving TFT often requires a considerably long channel. As a result, more chip area is required for forming the TFT, which limits the high resolution design of the TFT array substrate.